The invention relates to a method for treating coal to remove sulfur and ash.
Bituminous coal, generally found in the Eastern portion of the United States, contains a large amount of volatile matter, particularly sulfur and ash. Such coal can include as much as 6-8% total sulfur by weight. Inorganic sulfur, generally pyrites or iron sulfide, accounts for between 25% and 75% of the total sulfur content of bituminous coal.
Because of the high sulfur and ash content in bituminous coal, it is not practical to burn such coal on a commercial scale. Users cannot easily and economically comply with the clean air standards of the Environmental Protection Agency for SO.sub.2 emissions. Coal burning stationary plants are allowed 1.2 lbs. of SO.sub.2 emission per million BTU's of hot input according to recent EPA standards. Thus, the coal burned in such plants must contain no more than 1.5% total sulfur by weight. Further, bituminous coal must compete with anthracite coal, generally found in the Western portion of the United States, which contains 1% or less total sulfur by weight, well within the EPA standards.
Numerous methods have been proposed for removing all or a substantial amount of the sulfur and ash found in bituminous coal. Many of these methods require external applications of heat and/or pressure, some processes invoke so much heat that char forms. Other methods use sophisticated chemical reagents and equipment. These methods are expensive and, thus, are not economically feasible. Further, the processes create environmental problems in disposing of the by-products. Exemplary of these methods are the processes taught in U.S. Pat. Nos. 3,988,120; 4,162, 898; 4,167,397; 4,169,710; 4,226,601; 4,226,602; 4,256,464; 4,304,573; 4,305,815; and 4,332,593. DOE/ET Exploratory Study of Coal-Conversion Chemistry, Quarterly Report, SRI International (1981); DOE, Advanced Coal Preparation, Engineering Foundation (New York) 1981; Plaksin, New Methods of Increasing the Effectiveness of Concentration of Minerals. Academy of sciences of the U.S.S.R., A.A. Skochinskii Mining Institute (1973); Van Le, Floatability of Coal and Pyrite, M.S. Thesis Iowa State University (1977); Fisher et al., Advanced Development of Fine Coal Desulfurization and Recovery, Ames Laboratory, Iowa Sate University (1977); Hucko, Beneficiation of Coal by Selective Flocculation, a Laboratory Study, Bureau of Mines, Report of Investigations (1977).
U.S. Pat. No. 4,328,002 teaches a coal treatment process for desulfurization in which the coal is exposed to an oxidizing reagent/detergent solution and then exposed to a passivating/sequestering reagent and neutralized. The process has the advantage of creating a useful agricultural by-product. Furhter, the process does not require the application of external heat or pressure and the reactions are easily controlled and rapid. However, some of the possible oxidizing reagents are expensive, between two and four cycles of the process are necessary before the sulfur content of the coal is reduced below 1.5%, the process requires several different reagents, the pH of the solutions involved in the process oscillate between 2 and 9, the total reaction time is long, exothermic heating of the coal occurs and noxious odors are created.
Although several prior art methods of coal desulfurization have met with some acceptance, there is a need for a rapid, inexpensive method of desulfurization without the attendant disadvantages extant in the prior art.